Seems a big part of it was the software rendering of the filter: blur(5px) in Firefox, where other browsers are doing this in hardware. I've removed that CSS cl

Seems a big part of it was the software rendering of the filter: blur(5px) in Firefox, where other browsers are doing this in hardware. I've removed that CSS class for Firefox specifically now, and it's performing a lot better. The profiler now says 60% idle, 14% graphics, whereas previously there was no idle and it was 85% graphics.